It's just a framework, plain and simple. No special IDE or engines needed, just code.

It's open source and widely used, and has a strong community surrounding it

It's stable, mature, and actively developed for a JavaScript framework. Currently at vervsion 2.4.4 at the time of writing, with v3 in the works.

What really drew me towards Phaser was it's well documented API, and the numerous tutorials and examples from the community. Even though I'm just starting out, every feature that I've looked to implement appears to have been at least partially implemented and documented in a tutorial, making the learning curve much less steep than it could have been.

Powered by TypeScript, giving you the power of JavaScript with some extras

The only drawback I found was that it was really new when I was shopping for a game framework, which means that it's more on the bleeding edge than the cutting edge IMHO. Still, this is one I'll be watching.

PlayCanvas

I discovered PlayCanvas when I bought a Humble Bundle focused around game development tools. PlayCanvas seems to be cut from the same cloth as SuperPowers, but is a bit more mature considering it's been around longer. It focuses more on 3D content, although I'm sure you could do 2D content just fine.

Tooling runs in-browser, making it accessible to any development platform

Tooling was very easy to use, and I was able to start for free

PlayCanvas really reminded me the Unity3D toolset, which is widely used in the industry. Given, the value is really in the tooling, which isn't open source. Still, it might be worth checking out if you are looking for a toolset to go with your game engine.

Haxe

Okay, so Haxe really isn't JavaScript, but it's very similar to it and supports web development, so I'm including it. Plus, it really did stand out as a viable option to me when picking my game for a few reasons:

It's a language/toolkit that focuses on game / rich UI development

Very mature and has plenty of support around it for whatever games you'd like to develop

When targetting JavaScript, you can leverage other JavaScript libraries

Haxe is something that I keep coming across in my adventures with game development, and one day I'll head back and learn more about it. Just need to find the right project, and feel a bit more comfortable with game development.

Honorable Mentions

If I missed your favourite (which I'm sure I did) you should drop it in the comment section and share your favourite library and/or toolset. Extra points for open source repository links on the libraries.